This invention relates to a signaling extraction circuit which utilizes common control circuitry to carry out the signal extraction function for each of a plurality of digital groups, of time division multiplexed channels, that are time multiplexed together on to a common transmission link.
In the past, pulse code modulation (PCM) digital data terminals have typically performed the task of signaling extraction, as well as framing detection, reframing, etc., on a per "digroup" basis -- a digroup or digital group comprising a plurality of time division multiplexed PCM messages and multiplexed framing and signaling bits; see the article "D2 Channel Bank: Digital Functions" by A. J. Cirillo and D. K. Thovson, Bell System Technical Journal, VOl. 51, October 1972, pages 1701-1712, and the references cited therein. The per digroup partitioning of these functions has heretofore resulted in efficient terminal design.
With increasing digital traffic, it is not uncommon now to find proposals for multiplexing a plurality of digroups for transmission to a remote location over a common transmission facility or alternatively for multiplexing a plurality of received digroups on to a common bus at a switching center. These two cases are somewhat analogous and present the same problem with regard to signaling extraction. Conventional practice would suggest carrying out the signal extraction function on a per digroup basis using plural signal extraction circuits to respectively monitor the plurality of digroups. The obvious disadvantage of this approach is, of course, its complexity and costly redundancy in extraction circuitry.
It is accordingly a primary object of the present invention to carry out the signaling extraction function for each of a plurality of time division multiplexed digital groups in accordance with common control techniques.
In the D2 Channel Bank of the above-cited article, as well as in D3, eight bits are used for transmitting digital information (e.g., PCM encoded voice) for each channel in five of every six frames, and the eighth bit (D8) is borrowed for signaling purposes in every sixth frame. To identify these signaling digits, signaling framing information is inserted in the bit stream in the framing bit position of every other frame (i.e., the subframes). The subframe pattern that is used is . . . 111000111000111 . . . The signaling frame is defined as the frame which follows a transition in the subframe pattern. The algorithm used by the D2 and D3 receiving terminals to extract signaling information is to monitor the signaling subframe pattern for a transition (a 1 to 0 or 0 to 1) and to gate the D8 bits to signaling circuitry during the signaling frame that follows each transition. This transition algorithm is quite satisfactory in identifying the signaling frames under normal operating conditions. However, if the subframe pattern is disturbed (e.g., by excessive noise on the transmission line, by changes introduced into a digroup signal by a switching center for synchronization and/or reframe purposes, etc.) the signaling bits may not be properly identified and, as a result, disconnection or wrong number can occur.
It is therefore a further object of the invention to establish more positive or exacting criteria in the identification of signaling frames.
A related object of the invention is to implement a pattern recognition algorithm for the purpose of identifying signaling frames.
No provision is made with the D2 or D3 algorithm to "freeze" the signaling states of the channels during reframing. Instead, these states are allowed to vary randomly between on-hook (i.e., a binary 0) and off-hook (i.e., a binary 1) until framing has been re-established. The D2 - D3 algorithm thus relies upon the relationship between the reframing statistics and the normal office time-out to reduce the probability of disconnection or wrong number during reframing to a tolerable fraction.
It is another object of the invention to further reduce the probability of disconnection or wrong number by freezing the signaling states of the channels, of an out-of-frame digroup, during reframing.